High power parallel fiber arrays

ABSTRACT

High power parallel fiber arrays for the amplification of high peak power pulses are described. Fiber arrays based on individual fiber amplifiers as well as fiber arrays based on multi-core fibers can be implemented. The optical phase between the individual fiber amplifier elements of the fiber array is measured and controlled using a variety of phase detection and compensation techniques. High power fiber array amplifiers can be used for EUV and X-ray generation as well as pumping of parametric amplifiers.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 14/672,301 filed Mar. 30, 2015, which is a divisional application ofU.S. application Ser. No. 14/048,177 filed Oct. 8, 2013, now U.S. Pat.No. 9,013,786, issued Apr. 21, 2015, which is a continuation applicationof U.S. application Ser. No. 13/457,576, filed Apr. 27, 2012, now U.S.Pat. No. 8,736,954, issued May 27, 2014, which is a divisional of U.S.application Ser. No. 12/365,514, filed Feb. 4, 2009, now U.S. Pat. No.8,199,398, issued Jun. 12, 2012, which claims benefit of ProvisionalApplication No. 61/026,952, filed Feb. 7, 2008. The above-notedapplications are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to the field of ultra high peak power fiber lasersystems.

Efficient amplification in fiber amplifiers generally requires extendedamplifier lengths which result in substantial exposure of the fibers toself-focusing nonlinearities at elevated power levels. Indeed, it hasbeen shown that due to self-focusing, the obtainable peak powers infiber amplifiers are limited to around 5 MW.

In order to overcome the general nonlinear limitations of opticalfibers, multicore fiber designs (D. Scrifres, U.S. Pat. No. 5,566,196,Cheo et al., U.S. Pat. No. 7,107,795) have been suggested. Multicorefiber designs spread the signal intensity over a large core area andthus mitigate any nonlinear effects. In order to obtain near diffractionlimited output beams, it has further been suggested that passive lockingbetween the individual cores can be implemented (Scrifres '196 and CheoU.S. Pat. No. 6,031,850 and many others). Such passive phase lockingschemes can be implemented in a variety of ways, for example by settingup multi-core fiber lasers within the constraints of an optical cavity.Diffraction effects can then be used to minimize the loss of thephase-locked cavity supermode versus all other cavity super modes (e.g.M. Wrage et al., Opt. Lett., 26, 980 (2001); L. Michaille et al., ‘Phaselocking and supermode selection in multicore photonic crystal fiberlasers with a large doped area’, Opt. Lett., vol. 30, pp. 1668 (2005)).Passive phase-locking has also been demonstrated using fiber arrays,e.g. Shakir et al., U.S. Pat. No. 7,130,133 and Ionov et al., U.S. Pat.No. 6,882,781. These approaches are typically based on some type ofmode-selection which leads to preferential oscillation of certainsuper-modes compared to all other modes. Typically, these passivecoherent coupling techniques are based on cw laser signals. Moreover,the passive phase-locking approaches are difficult to implement and havelimited scalability. Passive phase-locking has also been suggested basedon nonlinear beam combination using photo-refractive materials inVerdiell et al., U.S. Pat. No. 5,121,400. However, photo-refractivematerials have significant power limitations and are therefore notuseful for high power applications.

As an alternative to such passive phase-coupling techniques,phase-conjugate mirrors have also been suggested to obtain neardiffraction limited modes from complicated arrangements of fibercouplers and multi-mode fibers (Betin et al., U.S. Pat. No. 6,480,327).However, no reliable methods of using phase conjugation for theconstruction of high power fiber lasers have yet been designed.

As an alternative to passive phase locking of multi-core fibers or fiberarrays, incoherent and coherent addition of fiber lasers has beenconsidered for overcoming the nonlinear limitations of single-coreoptical fibers. Incoherent addition is typically implemented viawavelength division multiplexing approaches using a linear array offiber lasers, where each fiber laser is designed to operate on adifferent wavelength to allow wavelength combination viawavelength-selective optical elements (see for example, T. Y. Fan,‘Laser Beam Combining for High-Power High-Radiance Sources’, IEEE J.Sel. Top. in Quantum Electronics, vol. 11, pp. 567 (2005). A limitationwith this technique is that it is typically restricted to very smallsignal bandwidths and is thus preferably implemented withsingle-frequency fiber lasers. In coherent addition typically tiled andfilled aperture approaches are distinguished as also described by Fan in(T. Y. Fan, ‘Laser Beam Combining for High-Power High-Radiance Sources’,IEEE J. Sel. Top. in Quantum Electronics, vol. 11, pp. 567 (2005)).

Coherent addition of multiple separate fibers is technically involvedand very expensive with limited potential for real commercialapplication. Coherent addition of such fiber arrays (typically referredto as fiber phased arrays (FPA)) has been demonstrated by several groups(E. Bott et al., U.S. Pat. No. 5,694,408; Rice et al., U.S. Pat. No.5,946,130);

Brosnan et al., U.S. Pat. No. 6,366,356; Johnson et al., U.S. Pat. No.6,233,085; M. Minden, U.S. Pat. 6,400,871; Rice et al., U.S. Pat. No.6,597,836; Rice et al., U.S. Pat. 6,678,288; M. Wickham et al., U.S.Pat. No. 6,708,003; R. Rice et al., U.S. Pat. No. 6,813,069; R. Rice etal., U.S. Pat. No. 7,065,110; T. Shay et al., U.S. Pat. No. 7,187,492;Rothenberg et al., U.S. Pat. No. 7,120,175; Rice et al., U.S. Pat. No.7,221,499 and S. Augst, ‘Coherent beam combining and phase noisemeasurements of ytterbium fiber amplifiers’, Opt. Lett., vol. 29, pp.474 (2004)). All these systems were based on the tiled aperture approachand borrow heavily from phase control techniques developed forastronomy, i.e. J. W. Hardy et al., ‘Real-time atmosphericcompensation’, J. Opt. Soc. Am., vol. 67, pp. 360 (1977) and T. R.O'Meara, ‘The multidither principle in adaptive optics’, J. Opt. Soc.Am., vol. 67, pp. 306 (1977). In astronomical applications, theatmospheric phase-front perturbations of an optical imaging system arecompensated by dividing a large phase front into several independentsections and using adaptive mirrors and heterodyne type phase detectionto stabilize the phase front in each individual section. Withcommercially available adaptive mirrors, phase-front perturbations canbe compensated for atmospheric fluctuations with bandwidths up to thekHz regime. The multidither type phase control techniques lendthemselves to tiled aperture coherent addition, however, to date none ofthe above references has demonstrated a filled aperture FPA.

Coherent addition in filled aperture configurations has been describedby Fan in (T. Y. Fan, ‘Laser Beam Combining for High-Power High-RadianceSources’, IEEE J. Sel. Top. in Quantum Electronics, vol. 11, pp. 567(2005)) and subsequently also by Rice et al., in U.S. patent applicationSer. No. 11/361,352. However, the system in ‘352 describes coherentaddition of cw amplifiers and relies on heterodyne phase detectiontechniques with relatively large feedback loop bandwidths.

Indeed, the adoption of phase front correction techniques as known fromastronomy to the phase control of FPAs has so far not been possible dueto the very large bandwidth of the phase fluctuations observed intypical fiber amplifiers, which can produce noticeable phasefluctuations at frequencies up to 10-100 kHz (see for example S. Augst,‘Coherent beam combining and phase noise measurements of ytterbium fiberamplifiers’, Opt. Lett., vol. 29, pp. 474 (2004)). Therefore phasecontrol in FPA is generally performed with phase locked loops withheterodyne phase detection techniques with feedback loop bandwidths inthe MHz range, which leads to the requirement for expensiveacousto-optic frequency modulators which have to be incorporated intoeach independent beamlet to ensure appropriate phase control.

Moreover, coherent addition of FPA has mostly been demonstrated with cwfiber amplifiers seeded with narrow bandwidth cw laser sources andcoherent addition of pulsed sources has had many limitations. Forexample in the work by Bott et al., U.S. Pat. No. 5,694,408, Bott onlyconsidered a tiled aperture system and no means were suggested for thereduction of nonlinearities in the fiber amplifiers when amplifying fspulses. In the work by Palese et al., U.S. patent application Ser. No.09/808,330 a pulsed source with a broad spectral bandwidth wasspectrally split into a linear array of channels and each channel wasamplified in an individual component of a fiber amplifier array.Subsequently the amplified spectral channels were recombined in adispersive optical element. A limitation with this approach is thelimited spectral filling fraction that is possible in the spectralsplitting and recombination process.

In yet another work, (ref. E. Cheung et al., ‘Phase locking of a pulsedfiber amplifier’, Opt. Soc. Conf. on Advanced Solid State Photonics,paper #WA2, (2008)) an amplitude modulated cw beam was coherently lockedto a non-modulated cw beam, which limits the spectral bandwidth of sucha scheme and the obtainable pulse widths.

In yet another example, (Mourou et al., in ‘Optical Pulse Amplifier withHigh Peak and High Average Power’ in International Publication No. WO2007/034317) coherent addition of pulses in fiber arrays is suggested,however no workable schemes for phase control of pulse fiber amplifierswere suggested. For example it was suggested to use the beat signalobserved when interfering two time delayed chirped pulses for phasecontrol. However, such a beat signal is only observable when the pulsesoverlap in time and especially for low repetition rate pulse sourcesthis greatly complicates phase detection

Coherent addition without spectral bandwidth limitation has beendescribed in a coherently multiplexed FPA based on individual isolatedfiber amplifier arrays or multicore fibers by Hartl et al., as disclosedin copending U.S. patent application Ser. No. 11/546,998, assigned tothe assignee of the present invention. The disclosure of Ser. No.11/546,998 is hereby incorporated by reference in its entirety.

As an alternative to the use of multi-core fiber to overcome the powerlimitations of optical fibers, highly multi-mode fibers have also beensuggested. These multi-mode fibers have very large mode areas and thushigh power signals can be propagated with much lower optical intensitycompared to single-mode fibers. Using adaptive control of the input modeto such multi-mode fibers, the excitation of a single principle mode ispossible using for example genetic algorithms for input control [H. Itohet al., Temtosecond pulse delivery through long multi-mode fiber usingadaptive pulse synthesis', J. J. Appl. Phys., 45, 5761 (2006); X. Shenet al., ‘Compensation for multimode fiber dispersion by adaptive optics,Opt. Lett., 30, 2985 (2005)]. Such principle modes are stable overextended periods of time even in km length fibers (in the range ofhundreds of ms), therefore relatively slow adaptive control based ongenetic algorithms can be implemented to find the principle modes andadjust the fiber launch mode to track a given principle mode with time.Though these schemes can compensate for modal dispersion in multimodefibers, principle modes in a multimode fiber are generally notdiffraction limited and of limited utility in high power laserapplications.

Another method for expanding the power limitations of fiber technologyhas been the implementation of external enhancement cavities which canincrease the power from a fiber amplifier by 1000-10,000 times usingadaptive phase control between the enhancement cavity and a fiberamplifier (I. Hartl et al., U.S. patent application Ser. No.11/546,998). The disadvantage of this technique is that it is generallyvery difficult to extract the optical power from an enhancement cavitywithout seriously affecting the possible cavity Q and the dispersiveproperties of the cavity.

Yet another method for expanding the peak power limits of fibertechnology has been the implementation of parametric amplificationschemes as recently described by Imeshev et al. in U.S. patentapplication Ser. No. 11/091,015. In such schemes a quantum amplifier isimplemented as pump for the parametric amplifier. Generally a pulsestretcher can be inserted in front of the quantum amplifier to avoidB-integral problems in the quantum amplifier. The stretched pulsesamplified in the quantum amplifier can further be compressed beforebeing directed for pumping the parametric amplifier. For maximum utilityin commercial applications of the system discussed by Imeshev in '015the quantum amplifier can further be based on a fiber system. However,to date no system configuration allowing the use of multi-core fibersfor the pumping of parametric amplifiers were described.

SUMMARY OF THE INVENTION

The present invention relates to the design of ultra-compact,high-power, high energy optical pulse sources and their applications.

In a first embodiment, near diffraction-limited high energy pulses aregenerated using chirped pulse amplification in coherently combinedarrayed fiber amplifiers.

In a second embodiment, coherent addition of individual fiber amplifiersis simplified by implementing an amplifier array in the form of amulti-core fiber. The utilization of strong thermal coupling of theindex fluctuations in the individual cores of multi-core fibers reducesthe bandwidth of any phase fluctuations inside the fiber to levelscontrollable with adaptive optics with kHz level feedback bandwidths.Fiber mode coupling and a resulting power exchange between individualfiber cores is minimized by using well separated cores with minimalspatial overlap.

Arrays of fibers with tens of individual members can be implemented,allowing for the generation of pulses with peak powers 10-100 timeshigher than the self-focusing limit of optical fibers at average powersabove the 100 W range.

The fiber arrays are fully compatible with cladding and side-pumpingschemes and therefore power scalable with achievable output powerslimited only by thermal considerations.

Fiber arrays based on polarization maintaining fibers can be implementedor alternatively, Faraday rotators in conjunction with double-passschemes can be implemented to minimize polarization fluctuations at theoutput of the fiber arrays.

The utilization of multi-core fibers greatly reduces the complexity offiber phased arrays, which is required to make them economicallyfeasible for general use.

In a third embodiment, multi-core fiber arrays can be designed withdensely packed fiber cores via the utilization of fibers with modeconfinement via air-holes or general low index leakage channels.Alternatively, multi-fiber arrays can be used which can be split at thesignal coupling end while being interconnected at the pump coupling endin order to minimize the optical complexity of the pump coupling scheme.Such strongly coupled multi-fiber arrays also greatly reduce thebandwidth requirements for coherent addition.

In a fourth embodiment complexity reduction is further enabled by theutilization of coherent spatial mode conversion techniques for efficientcoupling of optical signals into fiber arrays. The Strehl ratio ofcoherently combined fiber arrays is also maximized using coherentspatial mode conversion techniques at the output of the fiber phasedarrays and for coherent beam combination in a filled apertureconfiguration.

In a fifth embodiment the schemes for phase detection and control forfiber phased arrays according to the above embodiments are described.Optical phase control can be conducted with heterodyne phase detectionin various configurations.

For example heterodyne phase detection can be implemented with areference arm configured to interfere with the individual elements ofthe fiber phased array. To enable heterodyne phase detection, thereference arm is typically phase modulated at a frequency which ishigher than the bandwidth of the random phase fluctuations between theindividual elements of the phased array. The optical interferencepattern is then detected with a detector array and phase detectionelectronics is used to control the optical path length in each arrayelement using appropriate transducers, which are optically linked to thephased array.

Various options for appropriate transducers for control of the opticalpath lengths are possible, such as a mirror array operated inreflection.

Also various options can be implemented for phase detection. For exampleto control the optical phase of each fiber array element for pulses withlow repetition rates, additional cw lasers at a wavelength differentfrom the operation wavelength of the fiber amplifier array can becoupled into each array element and the whole phase detection can beperformed at the wavelength of the cw lasers. Equally, the phase ofoptical leakage signals incorporated between the pulses can be used forphase detection.

Alternatively, optical dithering of appropriate transducers can beimplemented to modulate the optical path length of each individual arrayelement. Phase sensitive detection of the optical interference patternbetween a non-modulated reference arm and the output of the fiber arraycan then be used to control the optical path length of each fiber arrayelement.

Optical phase control can also be conducted based on adaptive opticsschemes.

Optical phase control can also be implemented based on fast geneticalgorithms to find and track the phase coherent supermode of a fiberphased array based on multicore fiber structures using the maximizationof the Strehl ratio as optimization target.

The adaptive optics scheme can also be combined with digital holographyschemes to compensate phase fluctuations between the fibers using aspatial light modulator which generates the complex conjugate of thephase fluctuations inside the fiber and adds those to the signaltransmission path.

The embodiments described here can be used for high power machiningapplications, pumping of optical parametric amplifiers as well as forplasma, VUV, EUV and X-ray generation, or generally, where power scalingis desired but otherwise limited by nonlinear effects or device damage.

In a sixth embodiment, a parametric amplifier pumped by a pulsedcoherently combined multi-core fiber or general fiber phased array isdescribed.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present invention will become more apparent by describingin detail exemplary, non-limiting embodiments thereof with reference tothe accompanying drawings. The drawings are for illustrative purpose,and not to scale.

In the drawings:

FIG. 1 is diagram of a generic scheme for the amplification of highenergy pulses in a fiber phased array.

FIG. 2 is a diagram of a generic scheme for the utilization ofmulti-core fibers for the generation of near diffraction limited opticalbeams via coherent addition.

FIG. 3A is a cross sectional view schematically illustrating a multicorefiber comprising individual fiber cores based on step index fiber.

FIG. 3B is a cross sectional view schematically illustrating a multicorefiber comprising individual fiber cores based on leakage channel fiber.

FIG. 3C is a cross sectional view schematically illustrating a multicorefiber comprising individual fiber cores based on polarizationmaintaining leakage channel fiber.

FIG. 4 is a plot illustrating the radial modal intensity distribution ofa single-mode step index fiber (dotted line) and a typical leakagechannel fiber (dashed line and dashed-dotted line); the dashed linecorresponds to an intensity distribution along two low indexcapillaries, whereas the dashed-dotted line corresponds to an intensitydistribution along two gaps.

FIG. 5 is a plot of a side-pumping scheme for a multicore fiber.

FIG. 5A schematically illustrates a coax multicore fiber.

FIG. 6A is a diagram of a generic scheme for the coherent addition ofthe output from the individual fibers from a multicore fiber amplifieroperated in reflection.

FIG. 6B is an exemplary schematic illustration for seed signal injectioninto a multicore fiber amplifier. The same scheme can also be used forsignal extraction from a multicore fiber amplifier when coherentlyadding the individual cores in a filled aperture configuration.

FIG. 7 is a schematic illustration of a commercially available mirrorarray.

FIG. 8 is a diagram of a generic scheme for the coherent addition of theoutput from the individual fibers from a multicore fiber amplifieroperated in transmission.

FIG. 9 is a diagram of a generic scheme for injection seeding of amulticore fiber using a taper at the signal launch end.

FIG. 10A is a diagram of a phase controlled multicore fiber amplifierusing a cw laser for phase control.

FIG. 10B is a diagram of a phase controlled multicore fiber amplifierusing heterodyne phase detection by modulation of the reference beam.

FIG. 10C is a diagram of a phase controlled multicore fiber amplifierusing heterodyne phase detection by modulation of the beam path of eachindividual core.

FIG. 10D is a diagram of a phase controlled multicore fiber amplifierusing frequency combs.

FIG. 10E is a diagram of a phase controlled multicore fiber amplifierusing an array of interferometric cross correlators.

FIG. 11 is a diagram of a phase controlled multicore fiber amplifierusing an acousto-optic modulator both for pulse modulation and phasecontrol.

FIG. 12A is a diagram of a pump coupling scheme for a multicore fiberarray.

FIG. 12B is a diagram of geometric bundling of individual fibers of afiber array.

FIG. 13 is a diagram of a parametric amplifier according to an anotherembodiment.

DESCRIPTION OF THE EMBODIMENTS

A design example according to a first embodiment is shown in FIG. 1.System 100 comprises a high power fiber phased array system for thegeneration of high power optical pulses. The pulses are generated inseed source 101, which can comprise a diode, fiber or solid state laser.The pulses can have a pulse width from the fs range to about 1 μs. Thesecan further be stretched in time using dispersive optical elements.Elements for dispersive pulse stretching are well known from chirpedpulse amplification systems and are not further discussed here. Theoptical pulses from seed source 101 are distributed into individualoptical beam paths via coupler 102. Here an arrangement with a fiberoptic coupler 102 is shown, but alternatively, bulk optical beamsplitters or bulk diffractive elements could also be used to generate anarray of optical beam paths. Phase control elements, for examplemodulators 103-105, are then inserted into the beam paths to enablephase control of the individual optical beam paths. The pulsespropagating in the individual beam paths are then coupled into an arrayof fiber amplifiers 106-108. In order to achieve high output powers, thefiber amplifiers are typically double clad and any conventional pumpcoupling technique can be used for coupling pump light into theseamplifiers. The output of the fiber amplifiers is combined optically viaelement 109, which can comprise a lens array as well as combinations oflens arrays and diffractive optical elements. At the output of thesystem a near diffraction-limited optical beam 110 is generated, wherethe phase control enabled via modulators 103-105 is used to optimize theStrehl ratio of the output beam 110. Additional bulk optical elementscan be incorporated downstream of beam 110 for dispersive pulsecompression. Such elements are well known from chirped pulseamplification systems and are not further discussed here.

A design example according to a second embodiment is shown in FIG. 2.System 200 comprises a fiber phased array in the form of a multi-corefiber. Seed source 201 is split by optical element 202 into a numberindividual beams. The individual beams are passed through an array ofphase control elements, (e.g.: modulators 203), which allow forindependent phase control of each individual beam. The phase modulatedbeams are subsequently coupled into multi-core fiber amplifier 204,which comprises a number of individual cores 205. Here seven cores 205are shown, but a higher number of cores is also possible. The multi-coreamplifier 204 is typically double-clad, allowing for pumping with highpower semiconductor lasers. Any conventional pump coupling technique canbe used for coupling pump light into multi-core amplifier 204. Theoutput of the multi-core fiber amplifier 204 is passed through opticalbeam shaping element 206, which can comprise an array of lenses anddiffractive optical elements. At the output of the system a neardiffraction-limited optical beam 207 is generated, where the phasecontrol enabled via modulators 203 is used to optimize the Strehl ratioof the output beam 207. Additional bulk optical elements can beincorporated upstream of seed source 201 and downstream of beam 207 fordispersive pulse stretching and compression. Such elements are wellknown from chirped pulse amplification systems and are not furtherdiscussed here.

A specific design example of the cross section of a multi-core fiberaccording to a third embodiment is shown in FIG. 3a . It comprises afiber rod with a cladding diameter of 330 um and an isometric array of19 single-mode fibers. The outside fiber diameter in this example is 400μm. The core diameter of each individual core is 30 um and the core tocore spacing is 60 um. A variety of core designs can be implemented,i.e. conventional step index fiber designs, micro-structured fibers (L.Michaille et al., ‘Phase locking and supermode selection in multicorephotonic crystal fiber lasers with a large doped area’, Opt. Lett., vol.30, pp. 1668 (2005)). as well as leakage channel fibers (U.S.application Ser. No. 11/134,856 to Dong et al.) and Bragg fibers. Allthese fiber design are well known in the state of the art and notfurther described here. In all these designs Yb doping (or anotherrare-earth dopant) can be readily implemented in the core region toenable signal amplification. A thin low index cladding material for pumpguiding can further be implemented. The combined core area of this fiberis around 13400 μm² and approximately 3 times larger than the largestpossible core area of conventional large core fibers. The cladding/corearea ratio is of the order of 6.4; therefore very high claddingabsorption can be achieved in such a structure at low Yb doping levels,which greatly simplifies the manufacturing of this structure andincreases the fiber life time.

An example of the cross section of a multi-core fiber with 19 individualsingle-mode cores based on leakage channel fibers is shown in FIG. 3b .Here the shaded circles represent the core areas, which are doped withYb whereas the small non-shaded circles represent air-holes or glassareas with reduced refractive index. The design of each individual corewith its corresponding leakage channels follows the designconsiderations as disclosed by Dong et al. in copending U.S.applications Ser. No. 11/134,856, Ser. No. 60/975,478, Ser. No.61/086,433, and PCT international application no. PCT/US/74668, eachentitled “Glass Large-Core Optical Fibers, and assigned to the assigneeof the present invention. The disclosures of Ser. Nos. 11/134,856 ,60/975478, 61/086,433, and PCT/US/74668 are hereby incorporated byreference in their entirety.

The air hole size or the size of the area with reduced refractive indexis chosen to provide acceptable loss for the fundamental mode whileproviding a high loss for higher order modes. In an exemplaryembodiment, ytterbium-doped rods with a refractive index closely matchedto that of fused silica glass are hexagonally stacked with a second typeof rods, which may have the same diameter, so that each ytterbium-dopedrod is surrounded by six rods of the second type. The ytterbium rod canhave an ytterbium-doped center portion surrounded by fused silica glass.The second type of rods has a center portion with lower refractiveindex, e.g. fluorine-doped silica, further surrounded by fused silicaglass. The ratio (ytterbium rod diameter)/(second rod type diameter) istypically between 0.6 and 0.9. The hexagonal stack is typically insertedinto a silica glass tubing with an inner diameter slightly larger thanthat of the outer dimension of the hexagonal stack. In one embodiment,the resulting preform is drawn on a fiber drawing tower to anappropriate fiber diameter with the inside of the outer tube vacuumed. Alower index polymer coating can also be put on the fiber so that pumplight can guide in the glass area of the fiber. In an alternativeembodiment, a layer of capillaries are put between the stack and thesilica tube so that the pump can guide inside the layer of air holes. Ina further alternative embodiment, capillaries can be used in place ofthe second rod type with a fluorine-doped center portion. In anotherembodiment, boron-doped silica rods can be used in as illustrated inFIG. 3c to make each leakage channel fiber core birefringent forpolarization-maintaining applications. Examples of such polarizationmaintaining, multicore, leakage channel configurations will be furtherillustrated below.

The use of multicore leakage channel fibers allows tighter packing ofthe cores compared to conventional step-index multicore single-modefibers with less mode-coupling because of the minimization of the modalwings of each individual mode. This is further illustrated in FIG. 4,where the modal intensity distribution of a conventional single-modefiber is compared to the intensity distribution of a leakage channelfiber. Clearly the wings of the intensity distribution of the leakagechannel fiber go down to zero much faster than in conventionalstep-index fibers.

In the example shown in FIGS. 3b and 3c there is one low index featuredisposed between two adjacent rare-earth doped core regions. In order tofurther reduce mode-coupling, the core separation can be increased andmore than one low index feature can be disposed between two doped coreregions.

With leakage channel fibers, core diameter/core separation ratios ˜0.5can be achieved with minimal coupling between individual cores. Whencoherently adding all the emission patterns of such leakage channelbased multicore fibers, Strehl ratios of the far field emissionpattern >0.4 in a tiled aperture configuration can be achieved withoutthe use of any coherent mode modifying elements such as phase plates.Here we recall that the Strehl ratio is the ratio of the far fieldintensity of a beam with a certain intensity and phase distributionwithin an aperture over the far field intensity of a hard aperture beam.Strehl ratios approaching unity can be obtained using filled apertureconfigurations as explained below. Even in filled apertureconfigurations tight packing of the individual cores in multi-corefibers is beneficial as it allows a minimization of the overall fiberdiameter and improved heat dissipation compared to a fiber with a largerouter diameter.

Any tight packing of cores in a multicore structure leads to theformation of supermodes and mode-coupling between the cores. Inherentindex fluctuations inside the multicore structure may be caused byvarious physical mechanisms, for example stress or built-in refractiveindex variations. As a result, supermodes can be greatly suppressed. Themodes in the optical fiber can be represented as a simple linearcombination of the individual core modes with negligible mode-couplingbetween them.

In order for mode-coupling to be negligible, energy coupling betweenfiber array elements less than about 1% is preferred, and morepreferably smaller than 0.1%, or smaller than 0.02%. In one experimentby the inventors about 0.01% coupling was observed.

Supermode-suppression works most effectively for large core fibers,where the core diameter is >30 μm. In comparison to individual largecore fibers, where refractive index fluctuations limit the achievablemode size, in multicore fibers refractive index fluctuations are indeedbeneficial as they allow increased core stacking densities and largereffective mode areas compared to individual large core fibers.

Multicore fiber lasers can also be manufactured in an all polarizationmaintaining (PM) configuration. An exemplary design of a multicore PMfiber is shown in FIG. 3c . Here a leakage channel fiber is shown. Thefiber is very similar to the structure shown in FIG. 3b , but for theadditional incorporation of stress producing regions, which arerepresented by the small shaded areas disposed opposite each core inFIG. 3c . In this example the stress producing regions correspond to twoof six features immediately surrounding each core. The regions producestress in the fiber core and lead to polarization maintaining operation.PM single-core PM leakage channel fibers were discussed in Dong et al.in U.S. application Ser. No. 11/134,856; U.S. provisional application60/975,478; Ser. No. 61/086,433; and PCT international application no.PCT/US/74668, each entitled “Glass Large-Core Optical Fibers MulticorePM leakage channel fiber designs scale these polarization maintainingembodiments to multiple cores and are therefore not further discussedhere.

Multicore fibers can also be side-pumped as shown in FIG. 5, which showna side-pumping arrangement 300 for a multi-core fiber 301. Multicorefiber array 301 comprises a larger diameter solid-glass rod. V-grooves302 and optionally 303 are then cut into the side of the multi-corefiber. The multi-core fiber comprises individual cores 304-306; onlythree cores are shown but larger numbers of cores are also possible.V-grooves 302 and 303 are used to direct pump light into the multi-corefiber structure, where the pump light is designated with the arrows 307and 308. This pumping scheme is very similar to the those described forsingle core fibers in U.S. Pat. No. 5,854,865 entitled “Method forcoupling light into cladding-pumped fiber sources using embeddedmirrors”, and U.S. Pat. No. 6,704,479 entitled “Method and apparatus forside pumping a fiber”. These and various other pumping configurationsare well known for single-core fiber pumping and therefore not furtherdescribed here. FIG. 5 serves only as an example of a side-pumpingscheme; in principle any side-pumping scheme as used in context withsingle-core double clad fibers can also be used.

For an Yb glass multi-core fiber with a length of 1 m, the thermal loadfor an output power of 1 kW is calculated as around 50-100 W, which inturn produces a temperature differential of around 10° C. between thecentral and peripheral core regions. The corresponding optical pathlength difference between the central and the peripheral cores is thusaround 110 μm at full thermal load, corresponding to a time delay of 0.3ps. For near bandwidth-limited ns length pulses this time delay does notneed to be compensated as long as the coherence time is much greaterthan about 1 ps. For fs or strongly chirped ps and ns pulses, thethermally induced optical path length difference needs to becompensated. This can be achieved by the introduction of appropriatephase delays introduced before or after the fiber. Such phase delays canfor example be implemented with optical phase plates of a certainthickness. For small heat loads an adaptive optics compensation schemecan adjust for the path length difference.

Alternatively the cores can be located in a single ring at the peripheryof the fiber as exemplified in FIG. 5a . In various embodiments allcores have approximately the same radial separation from the fibercenter, and the thermally-induced optical path length differencesbetween all cores is approximately equalized. In the following we referto such a structure as a coax multicore fiber. Moreover, locating allcores on the periphery in a coax multicore fiber allows for moreefficient cooling and operation of the coax multicore fiber atabsorption levels beyond the stress fracture limit of a conventionalglass rods. The reason is that the temperature increase in the center ofa coax multicore fiber is significantly lower for the same heatloadcompared to a conventional glass rod for the same heat load per meter.To operate at very large levels of heat load it is therefore beneficialto confine the pump light also in a ring at the periphery of the fiber.This can for example be accomplished by using a low index material forthe central fiber area, such a fluorine glass. Numerous variations ofthe coax multicore fiber are possible where the cores are disposedsymmetrically about the fiber center. In some embodiments the fibercores may be disposed at the vertices of a regular polygon, for examplea polygon having 6,8,12, or more sides. Features may be disposed abouteach core and may comprise air holes or a low index glass. Polarizationmaintaining configurations similar to that of FIG. 3c . may be used insome embodiments.

An exemplary implementation of a multicore fiber amplifier according toa fourth embodiment in a set-up 400 for simultaneous phase control inall individual cores is shown in FIG. 6a . Here a multicore fiber asshown in FIGS. 3a and 3b is used. In one basic implementation the frontend of an optical near diffraction limited beam from a laser seeder 401is imaged via a phase plate 402 (or diffractive optical element) ontothe front facet of a multi-core fiber amplifier 403. The phase mask 402is used to transfer the single beam pattern from the seed laser into amulti-beam pattern on the surface of the multicore fiber 403 and toconcentrate the light from the front end system to the location of eachindividual core of the multi-core amplifier. In this example, a fractionof the seed light is directed with beam splitters (BS) 404, 406, andmirror 405 to a detector array 407 which is used for phase detection. Inprinciple more than one seed laser with a corresponding multi-beampattern can also be implemented. For stable operation of the beamtransfer from seed to multi-core amplifier via a phase plate, anymultiple seed beams need to be coherent. System implementation withmultiple seed beams is a straightforward extension to FIG. 6a and arenot further discussed here.

In order to avoid feedback from the multicore amplifier, an isolator(not shown) is typically inserted after the seeder 401. The seeder light401 is amplified in each individual core of the multi-pass amplifier,where a double-pass configuration including a Faraday rotator mirror 408is used to maximize the signal gain and to compensate for anypolarization drifts inside the assembly.

In this example an end-pumped configuration is shown. The pump lightfrom pump laser 409 is provided via a dichroic beamsplitter 410 andpolarization beam splitter 411 inserted at the signal launch end. Hereit is assumed that the pump and seed light have opposite polarizationstates. Appropriate optics upstream of the dichroic beamsplitter isfurther used to maximize the coupling efficiency of both the pump andsignal beam. The pump is conveniently obtained from a beam-shapedsemiconductor laser (see for example Fermann et al., U.S. Pat. No.6,778,732 and references therein) and coupled into the pump cladding ofthe multicore fibers. Alternatively, side-pumping schemes as describedwith respect to FIG. 5 can be implemented which further simplifies theassembly. Equally the assembly can further be simplified whenincorporating integrated components instead of bulk optics components inthe present embodiment.

An adaptive mirror or an adaptive mirror array 412 is inserted after thefirst pass through the multicore fiber to modulate and control theslowly varying phase between individual fiber cores. The mirror arraycan for example be constructed from piezo-electric transducers as wellas MEMs arrays. A lens pair 413, 414 is further implemented to image theoutput of the multi-core fiber 403 onto the mirror array 412. The outputof the multi-core fiber rod 403 is directed via the polarization beamsplitter 411 onto a second phase plate (not shown) similar to the firstphase plate 402 for beam combination and then to the application. Asmall fraction of the output beam is directed via beam splitter 406 ontodetector array 407. Phase information for the optical pathscorresponding to each individual core within the multicore fiber isobtained by interfering a fraction of the seed signal with a fraction ofthe output from the multi-core fiber 403.

The signal from the seed laser may be dispersively stretched in time andan additional bulk dispersive pulse compression element may be includedto further increase the peak power of the pulses.

An example of a commercial adaptive mirror array is shown in FIG. 7. Forwell designed mirror arrays, the location of each mirror can be adjustedat a frequency between 100-1000 Hz, which is sufficient to compensatefor the slow phase fluctuations in the multi-core fiber array, whichhave the largest amplitudes in a bandwidth in the 1-100 Hz range, oncethe fiber is operated at constant temperature.

In various embodiments commercially available MEMs devices having anarray of spatially separated mirrors may be utilized for phase control.Each element of the MEMs array may include a mirror controllable over alength of several microns along the optical axis (stroke length), andmay provide for tip/tilt control. By way of example, the S37 seriesavailable from Iris AO, Inc. includes MEMs deformable mirrors, includingwith arrays having 37 elements, a maximum stroke of 12 μm, controllableup to about 2 KHz, with control software.

In various embodiments the required adaptive mirror electronic actuatorcontrol is performed using standard techniques as known from astronomy,i.e. a small dither signal is applied to each mirror and the phase ofthe optical signal path along that arm is measured using heterodynedetection at the dither frequency.

With this multi-core fiber array, the power limits of conventionalsingle-mode fibers can be exceeded by a factor of 10-100, where theaverage power capability can be in the kW range.

A specific design example according to a fourth embodiment comprising anoptical arrangement for coherent addition 500 is further discussed inFIG. 6b ). For simplicity, we assume that the multi-core fiber 501 isside-pumped and polarization maintaining. The output from a single-modefiber (not shown) is magnified with an appropriate telescope (also notshown) to produce an input beam 502 with a spot size diameter of 100 μmin a plane 503 located at point PO. The corresponding angular divergenceis thus 0.73 degrees and the corresponding numerical aperture of theinput beam is 0.0064 at a wavelength of 1 μm. A first lens L1 504 with afocal length of 100 mm is then used to collimate the beam from point PO.A phase plate 505 is positioned at a distance of 100 mm from lens 504 tosplit the input beam into a multitude of diffracted beams. The spot sizeon the phase plate is calculated from the divergence of the input beamas 1.3 mm. By selecting a modulation period of d=0.4 mm on the phaseplate we obtain a diffraction angle of sin(α)= 1/400=0.0025. Bypositioning the phase plate in the focus of a second lens L2 506 offocal length 40 mm, the single beam is transformed into an array ofbeams with spot diameter of 40 μm with a beam to beam separation of 100μm.

An appropriate multi-core fiber 501 designed for receiving the inputbeam can be a loss channel fiber (e.g.: a leakage channel fiber oranother multicore fiber of different design) with core diameters of 50μm and a core to core separation of 100 μm. The same arrangement canalso be operated in reverse to combine the output of a multi-core fiberinto a single beam, where lens L1 504 can be omitted. Equally, theoptical configuration shown in FIG. 6b operated in reverse can also beused for beam combination in a filled aperture configuration using onlya single pass through the multicore fiber array 501. Techniques forcontrolling the phase of each individual core in single-passconfigurations are discussed with respect to FIG. 8 below.

The multi-core fiber amplifier 501 can be isolated from a seed-beam byusing an isolator upstream of point P0. Also when operated in adouble-pass configuration, the output can be extracted by positioning ofa Faraday rotator and a polarization beam splitter upstream of point P0.Such optical elements are well known in the state of the art and notfurther discussed here.

In the configuration shown in FIG. 6b the second distal end of themulti-core fiber is imaged onto a mirror array, for example the arrayshown in FIG. 7. Since the individual facets of mirror arrays aretypically of the order of mm in diameter, appropriate magnifying opticscan be implemented to increase the separation of the individualbeam-lets from the multicore fiber on the mirror arrays. Optics forimage magnification are well known in the state of the art and are notfurther discussed here.

In a variation of the fourth embodiment, instead of a double-passarrangement, single-pass arrangements can also be implemented. However,single pass arrangements are slightly more complex and do notautomatically compensate for any polarization drifts inside theamplifier. For single-pass arrangements, therefore, it is convenient touse polarization maintaining multicore fiber arrays as shown in FIG. 3c. An exemplary single-pass arrangement 600 using a polarizationmaintaining multicore fiber 601 is shown in FIG. 8. Here a multicorefiber pre-amplifier 602, matched in dimensions to the final poweramplifiers 601 is shown which is used to spatially precondition thesignal beam for optimum coupling of the output from the mirror array tothe final power amplifier. Again the use of side-pumping schemes furthersimplifies the optical assembly. The optical arrangement can further besimplified by substituting the pre-amplifier multicore fiber with atapered multicore fiber with a single beam input as shown in FIG. 9.Here the multicore fiber 700 is tapered to a small diameter at thesignal input end 701. Such a multicore fiber is equivalent to a starcoupler, splitting an input signal approximately equally into allindividual fiber cores in the expanded region of the fiber. Anadditional coupler upstream of the tapered fiber region can further beused to provide the reference signal for the detector array.Alternatively, the mirror array 603 in FIG. 8 can be supplemented orreplaced with a spatial beam modulator operated in transmission. Aspatial beam modulator, for example a spatial light modulator (SLM), canfor example be inserted between the pre-amplifier 602 and the poweramplifier 601. Such spatial light modulators are well known in the stateof the art and are not further discussed here. The use of such spatialmodulators greatly simplifies the assembly shown in FIG. 8. A phaseplate (not shown) can be inserted at the output of the multi-core fiberamplifier for maximization of the Strehl ratio of the output beam.

The interference of the reference signal and the output of the multicorefiber 601 can further be used to provide feedback to the spatial lightmodulator to create the desired interference pattern that corresponds toall fiber cores being in phase. The desired interference pattern can bedetermined with a genetic algorithm. To obtain an appropriate costfunction for feedback to the spatial light modulator the differencebetween the desired and actual interference pattern may be calculated.

For more rapid phase control digital holography techniques can beimplemented. The principles of digital holography were discussed in U.S.Pat. No. 5,378,888 to Stappaerts et al. and in ‘Coherent fiber combiningby digital holography’, C. Bellanger et al., Opt. Lett., vol. 33, no.24, pp. 2937, December 2008. In order to implement digital holographyfor phase control a small test beam needs to be passed backward throughthe multicore fiber array and interfered with the reference beam on anadditional detector array (not shown). This can be done by usingappropriate arrangement of beam splitters, mirrors, and/or other opticalelements for spatially dividing or directing beams. The main beam isconfigured to pass through the spatial light modulator as before. Byfeeding the interference pattern between the test and reference beamback to the the spatial light modulator, the spatial light modulator canthen be configured to generate the complex conjugate of the interferencepattern when passing the main beam. Thus the phase fluctuations insidethe multicore fiber array can be compensated. The test beam and the mainbeam will have similar wavelengths for the phase compensation techniqueto work best. Preferably, the test beam has a small spectral bandwidthand has a wavelength centered within the spectral bandwidth of the mainbeam. Digital holography techniques are compatible with any of the fiberphased array configurations discussed here. Digital holographytechniques are well known in the state of the art and are therefore notfurther discussed here. Because of the low frequency of phasefluctuations in multicore fiber arrays digital holography techniques arevery efficient in compensating for phase fluctuations in suchstructures.

In a fifth embodiment, several additional schemes can be implemented forphase detection.

A preferred embodiment is shown in FIG. 10a . A pulsed seed “signal”light source 800 is imaged onto the cores of a multi-core amplifierfiber 806 using a phase plate as a diffractive element 801 and relaylenses 802 and 803. At the fiber end facet the fiber cores are imaged ona segmented mirror array 808 using relay lenses 804 and 805 such thatlight emitted from a single core is reflected back into the same core bya single mirror segment. Each mirror segment can be translated by anactuator parallel to the light propagation direction. A Faraday rotator807 is double passed to ensure environmental stability againstpolarization rotation in the amplifier fiber cores. After a secondbackward pass through the amplifier cores the light is separated fromthe launched light by the polarizing beam splitter 809. The multicoreamplifier fiber 806 is cladding pumped by the pump source 810.

Light from a narrow linewidth continuous wave “stabilization” laser 811is co-propagated through the individual doped fiber cores and used forphase detection. The wavelength of the light source is chosen to be ofhigh transmission through the fibers. Preferably the wavelength of thecw light source is chosen to be different from the signal wavelength ofthe seed source 800 and to be outside the maximum gain band of theamplifier fiber cores. This ensures that little or no gain is obtainedby the co-propagating cw light and the cw light does not significantlydeplete the amplifier gain. Preferably the wavelength of the cw lightsource is chosen close enough to the signal wavelength to not sufferfrom chromatic aberrations of the relay lenses 803, 804 and 805 and suchthat the diffraction angle at the diffractive element 801 is close tothat of the signal wavelength.

A portion of the cw light bypasses the active fibers and is used asreference beam. This part is frequency shifted by the frequency of alocal oscillator 813 using an acousto-optic modulator (AOM) 812 andilluminates the elements of a photo-detector array. The cw-lightco-propagating with the signal light is combined with the signal lightat beam-splitter 814. A lens 816 together with lens 803 is used to imageeach fiber core onto a single photodiode of the photodiode array 817. Ifthe wavelength of the cw laser is different from the seed laser anoptical bandpass filter, for example interference filter (IF) 815 and acoating on beamsplitter 814 can be used to prevent light outside thewavelength of the cw laser from saturating the detectors.

At the detector array elements the reference light of a co-propagated cwlight and the signal light interfere and a heterodyne beat signal isdetected. Optional waveplates, diffractive elements and additionallenses in the reference beam can be inserted to maximize the beatsignal. Any change in optical path-length of the active fiber coresleads to a Doppler shift of the co-propagating cw-light and therefore toa frequency shift of the beat signal. This beat signal can therefore beused to stabilize the optical path length of all fiber cores to aconstant value. This method is commonly used for combining CW lasers anddescribed for example in S. J. Augst et al. Opt. Lett. 29, 474 (2004).In the embodiment described here the cw laser is however used for pathlength stabilization of the individual fiber cores and is different fromthe amplified light. This embodiment can be used for all repetitionfrequencies of the seed source. For path length stabilization theheterodyne signal is optionally filtered by the band pass filters 818.Phase-detectors (PD) 820 are used to detect the relative phase betweenthe heterodyne beat signal and a reference oscillator. Thosephase-detectors provide the error signal for the feedback stabilizationloop which is closed by the loop filters 821 controlling the mirroractuators. Optional pre-scalers (PS) 819, for example a divide by 16circuit, can be used to increase the locking range of the feedback loop.

The dynamics of the feedback loop are determined by the frequencyresponse of the loop filters and the actuator element. The fastestresponse time of the loop-filter and actuators is on the 10 μs-timescalewhich is significantly longer than the seed pulse length. Therefore fastphase changes in the cw-laser which can for example occur due to crossphase modulation of the cw-laser with the amplified signal light is notinterfering with the phase compensating feedback loop.

In a second embodiment of phase detection the seed light itself can beused as reference as shown in FIG. 10b . Since the frequency of thelocal oscillator needs to be lower than the pulse repetition frequencybut high enough to allow efficient heterodyne detection, this embodimentis preferably used for pulse-repetition frequencies above 1 MHz. For lowfrequencies the RF mixer based phase detectors can be replaced byanalogue to digital converters (ADCs) and digital signal processing. Inthis case the frequency bandwidth of the feedback loop is preferablyselected to be less than one tenth of the pulse repetition frequency.

An alternative embodiment for phase detection at low repetition rates isshown in FIG. 11. Here a high repetition rate (>10 MHz) mode-lockedoscillator 900 is used as seed source. An acousto-optical modulator 902is used to modulate the intensity of the optical pulses from theseed-laser. This is done by applying an RF drive frequency to themodulator 902 and arranging the optical system to propagate theresulting 1^(st) order diffracted beam. The 0^(th) order non-diffractedbeam (not shown) is blocked so as to prevent the energy from propagatingthrough the optical system. These techniques are often used in thedesign of acousto optical modulator and deflector systems and notfurther discussed here. The RF driving field for the modulator isgenerated using the RF reference oscillator 904 and an RF amplifier 903which has an input for amplitude modulation. The RF amplifier is nowmodulated in a way that high power RF pulses are generated at asignificantly lower repetition rate than the mode-locked opticaloscillator but synchronized with every n^(th) oscillator pulse and atother times the RF power is significantly lower but not zero. In thisway the optical pulse train is modulated in a way that every nth pulseis of high intensity and other pulses are of significantly lower butnon-zero intensity. This enables use of the frequency shifted,diffracted pulse-train at the oscillator repetition rate for heterodynebeat detection with part of the oscillator light sampled by beamsplitter 901. The two intensities are chosen in a way that the storedamplifier energy is mainly depleted by the high intensity seed pulsesand the intensity contrast between high and low intensity pulses at theamplifier output is significant. To prevent saturation of thephase-detection electronics by the high intensity optical pulses,limiter circuits or fast electronic switches can be implemented. In thiscase the frequency bandwidth of the feedback loop is preferably selectedto be ten times higher than the pulse repetition frequency.

In a third embodiment of phase detection shown in FIG. 10c a smallmodulation or dither signal derived from the local oscillator is appliedto each element of the mirror actuator array. The signals from thedetector array are phase sensitively detected. After low pass filteringthe phase error is combined with the small modulation signal and used asfeedback signal to the actuator array. The cut-off frequency of the lowpass filter is lower than the dither frequency.

In a fourth embodiment of phase detection shown in FIG. 10d thereference beam is not shifted in wavelength and the pulse repetitionfrequency of the seed is used as local oscillator. This approachutilizes the frequency comb structure of the seed laser and is describedin detail in Yi-Fei Chen et al. “Remote distribution of a mode-lockedpulse train with sub 40-as jitter”. The work described by Chen et al.however stabilizes only the path length of a single optical fiberwithout gain. In the embodiment in FIG. 10d multiple fibers arestabilized to equal path length and optical gain is present.

In a fifth embodiment of phase detection shown in FIG. 10e the detectorarray is replaced by an array of interferometric cross-correlators. Thefringe pattern of the interferometric signal is detected andsoftware-based low bandwidth feedback stabilization is implemented. In amodification of this embodiment the signal from one active core is usedas a reference beam.

Numerous combinations are variations of the above example embodimentsare possible.

As an alternative to the fringe pattern generated by interferometriccross-correlators, also the fringe pattern from spectral interferencebetween the amplified signal pulses and a reference beam can be used forfeedback stabilization.

In some embodiments, instead of heterodyne detection techniques forphase control, genetic adaptive optic algorithms can also be implementedfor a minimization of the wavefront errors from multicore fiber arrays.Because genetic algorithms are typically much slower than heterodynedetection, these algorithms may be suited for use with stronglythermally coupled multicore fibers. In some embodiments, optimizedalgorithms and special purpose hardware may provide an increased controlbandwidth for systems based on genetic algorithms. The use of geneticalgorithms eliminates the need for interferometric detection of thephase fronts and a reference arm. A multicore fiber amplifier based onthe use of a genetic algorithm for phase control is thereforeconstructed very similarly to the design shown in FIG. 6a , where thereference arm is eliminated. To obtain an appropriate cost function forfeedback to the mirror array a frequency doubling stage for a fractionof the output beam is implemented. The frequency doubled power can thenbe measured with a single detector and optimized by appropriateadiabatic adjustment of the mirror array. Alternatively, a detectorarray can be implemented, which samples the frequency doubled beam atseveral locations; an appropriate cost function then maximizes the powerin the central beam part and minimizes the power in the peripheral partsof the beam. Clearly a side-coupled multicore amplifier with adaptivemodal control based on a genetic algorithm is highly compact and doesnot require many components, which is ideal for commercial systems.

Instead of multi-core fibers, more conventional fiber phased arrays canalso be used for coherent addition. Such arrangements are well known inthe state of the art and a system implementations to be used withoptical signals was discussed in U.S. patent application Ser. No.11/546,998 and is not further described here. Conventional fiber arrayscan use separate pump diodes for each array element, which increases thenoise bandwidth of the phase fluctuations. Therefore, it is beneficialto use acousto-optic phase modulators for each array element also.Because of the phase noise bandwidth of 10-100 kHz in this case, thepulse repetition rate has to be in the range of 100 kHz to 1 MHz inorder to allow phase control without a separate cw control laser. Forlower repetition rate signals the leakage between a pulse modulator asdiscussed above with respect to FIG. 10b can be used for phasemodulation.

Also for pulse repetition rates below 100 kHz, a cw reference signal canbe used to allow an adequate bandwidth for phase control, as alreadyexplained with respect to FIG. 10a . Such cw lasers are selected at awavelength of high transmission through the amplifier fibers (i.e. 1300nm for Yb amplifiers) and can be coupled into each array element toequalize the phase of each array arm. However, cw lasers increase thecomplexity of the system.

Coherent addition at pulse repetition rates >1 MHz becomes progressivelysimpler because of the ability to control phase fluctuations with anincreased bandwidth without the use of leakage signals.

When using modulator arrays for phase control of individual beam pathsin coherent addition, a reduction of the component count can beaccomplished by using one pump beam. Such an exemplary embodiment isshown in FIGS. 12a and 12b . FIG. 12a shows an assembly 1000 forcoherent addition of fiber amplifiers when using one pump beam. Here amulticore fiber 1001 is assembled from an array of individual fibers1002, 1003, 1004 which are loosely fused at their boundaries. The crosssection of such a loosely fused fiber array is shown in FIG. 12b .Because the fibers are loosely fused, they can be split into individualfibers at the signal injection end allowing coupling of individualsignal beams 1005, 1006, 1007 into each fiber, as shown in FIG. 12a . Atthe pump coupling end 1008, the fiber ends can be fused further to allowefficient coupling of a pump beam 1009 from pump 1010 into the fiberarray 1001 via beam splitter 1011 and lens 1012.

The Strehl ratio of the output beam can again be maximized by the use ofphase plates in a filled aperture configuration as discussed withrespect to FIGS. 6a and 6 b.

The above embodiments were shown based on mainly transmissive optics,such as lens and transmissive phase plates. Various embodimentsdescribed herein are directed to the design of high power laser systems,including both high peak and average powers. At power levels above 100W, thermal management can benefit greatly from the use of reflectiveoptics such as mirrors and diffractive elements operated in reflection.The replacement of lenses with mirrors and the replacement oftransmissive diffractive elements with reflective diffractive elementsis straight-forward and not further discussed here.

The pulsed, coherently combined fiber laser sources as described hereare ideal as pump sources for optical parametric amplifiers as well asfor high power EUV, X-ray and plasma generation. For EUV and X-raygeneration typically laser induced plasmas are used, where the plasma isgenerated by directing the coherently combined pulses onto solid orliquid metal targets. The increased peak power of coherently combinedpulsed fiber laser sources greatly improves the conversion efficiency ofEUV and X-ray generation compared to single-core fibers. High power EUVand X-rays sources are of great interest in advanced lithographyapplications and high resolution imaging and will benefit greatly fromthe implementation of compact high power coherently combined fiber basedsources as discussed here.

Compact high power parametric amplifiers were discussed in U.S. patentapplication Ser. No. 11,091,015 and are not further discussed here. Animplementation with a multicore fiber amplifier as parametric pumpsources is very attractive, since they increase the obtainable pulseenergy from such systems compared to pumping with single-core fiberamplifiers.

An example of a parametric amplifier pumped with a coherently combinedfiber laser 1100 is shown in FIG. 13 according to a sixth embodiment.Here a single-seed oscillator 1101 can be used for convenience. The seedoscillator is Yb based and seeds the multicore fiber amplifier 1102 togenerated pump pulses with an energy in the range from 10-1000 mJ. Theseed oscillator can be conveniently frequency shifted via passivefrequency conversion elements as discussed in '015 to seed theparametric amplifier crystal 1103. By temporally overlapping both theparametric seed signal and the output from the multicore fiberamplifier, efficient amplification in the parametric amplifier crystalcan be obtained. The B integral in the multicore fiber amplifier canfurther be minimized by the implementation of chirped pulseamplification schemes. In order to implement chirped pulseamplification, a pulse stretcher stage is implemented after the seedoscillator and before signal injection into the multi-core (ormulti-element) fiber amplifier. After coherent combination of theindividual beamlets of the multi-core (or multi-element) fiberamplifier, a pulse compressor stage is implemented. Such pulsecompressor stages are for example conveniently based on bulk diffractiongratings. After pulse compression, the pump pulses are further directedto the parametric amplifier, where they are used to amplify the seedsignal. With multi-core (or multi-element) fiber amplifiers high averagepower parametric amplifiers generating pulse energies in excess of 10 mJcan be generated.

Thus the inventors have described high peak power fiber amplifiersystems having at least one array of fiber amplifiers, and particularlyadapted for coherent combination of laser pulses. The amplifier systemsare applicable in high peak power, short pulse applications. Forexample, peak power on the order of at least 1 MW may be generated. Thesystems may be used for EUV or x-ray generation, optical lithography,laser radar, or similar applications.

At least one embodiment includes a high-peak power fiber amplifiersystem. The system includes an array of fiber amplifiers. The amplifiersof the array are disposed in such a way that thermal fluctuations of theamplifiers are sufficiently matched and limit relative phasefluctuations at amplifier outputs to a low-bandwidth, for example toless than about 10 KHz. The amplifiers are disposed at sufficientrelative distance such that energy coupling between any amplifiers isnegligible. The system includes a means for seeding the array ofamplifiers, including a laser source. The seed pulse and/or amplifiedpulses include pulse widths in the range of femtoseconds to about 1microsecond. A beam distributor is disposed between the laser source andthe array to distribute a pulse from the source, or to distribute apulse from a means for seeding. The pulse is distributed into aplurality of beam paths incident on corresponding amplifiers of thearray. The beams have a spatial distribution substantially similar tothe spatial distribution of the pulse. At least one pump source isincluded for optically pumping the fiber amplifier array. A plurality ofphase-control elements arranged in a spatial relation are opticallyconnected to fiber amplifiers of the array. The phase-control elementsmodify an optical phase of at least one fiber amplifier output inresponse to a phase-control signal. The system also includes a means forproducing a plurality of control signals applied to the phase controlelements so as to control the optical phase at the output of themajority of the fiber amplifiers. The control signal and phase-controlelements are configured to stabilize the optical phase between themajority of the individual fiber amplifiers of the array.

At least one embodiment includes a high peak power fiber amplifiersystem comprising an array of fiber amplifiers. The array is configuredin such a way that a spatial separation of the cores of said amplifiersis sufficiently small to provide strong thermal coupling that limitsoutput phase fluctuations of the array to a low bandwidth, for exampleless than about 10 Khz. A sufficiently large spatial separation betweenfiber amplifiers also limits optical energy coupling between amplifiersof the array, for example to about 0.1% or less. A plurality ofphase-control elements are arranged in a spatial relation and opticallyconnected to fiber amplifiers of the array. The phase-control elementsmodify an optical phase of at least one fiber amplifier output inresponse to a phase control signal. The system also includes a phasecontroller generating the phase control signals, and is operable tostabilize the optical phase at the output of amplifiers of the array.

At least one embodiment includes an amplifier system for coherentcombination of laser pulses. The embodiment includes an array of fiberamplifiers, for example a plurality of individual fiber amplifiers, andat least one pump source configured to optically pump the fiberamplifier array. A pulsed master oscillator seeds the fiber amplifierarray A plurality of phase-control elements arranged in a spatialrelation are optically connected to fiber amplifiers of the array. Thephase-control elements modify an optical phase of at least one fiberamplifier output in response to a phase-control signal. A means forproducing a plurality of control signals applied to the phase controlelements controls the optical phase at the output of the majority of thefiber amplifiers. The control signal and the phase-control elementsstabilize the optical phase between the majority of individual fiberamplifiers.

At least one embodiment includes an amplifier system for coherentcombination of laser pulses, for example sub-nanosecond pulses. Theembodiment includes an array of fiber amplifiers, for example aplurality of individual fiber amplifiers, and at least one pump sourceconfigured to optically pump the fiber amplifier array. A pulsed masteroscillator seeds the fiber amplifier array. A fraction of the output ofthe master oscillator is used in a reference arm, and configured tooptically interfere with a fraction of the output from the fiber array.Optical interference is detected with a detector array. The referencearm is further phase modulated to allow for heterodyne phase detectionof the optical phase of the majority of the elements of the fiber arraywith the detector array. A plurality of phase-control elements arrangedin a spatial relation are optically connected to fiber amplifiers of thearray. The phase-control elements modify an optical phase of at leastone fiber amplifier output in response to a phase-control signal. Aheterodyne phase detector and the phase control elements stabilize theoptical output phase between the majority of the individual elements ofthe fiber array.

At least one embodiment includes an amplifier system for coherentcombination of laser pulses, for example sub-nanosecond pulses. Theembodiment includes an array of fiber amplifiers, for example aplurality of individual fiber amplifiers, and at least one pump sourceconfigured to optically pump the fiber amplifier array. A pulsed masteroscillator seeds the fiber amplifier array. A fraction of the output ofthe master oscillator is used in a reference arm, and configured tooptically interfere with a fraction of the output from the fiber array.Optical interference is detected with a detector array. The opticalphase of the elements of a fiber amplifier array are dithered at afrequency derived from a local oscillator, allowing for heterodyne phasedetection of the optical phase of the majority of the elements of thefiber array with the detector array. The heterodyne phase detector isconfigured to stabilize the optical output phase between the majority ofthe individual elements of the fiber array.

In various embodiments:

-   -   amplifiers may be arranged such that thermal fluctuations in the        indices of refraction of the gain media of the amplifiers are        sufficiently matched so that relative phase fluctuations at        amplifier outputs are limited to the low-bandwidth, for example        less than about 10 KHz.    -   a control signal and a phase modulator may be configured for        maximizing the Strehl ratio of an output of a fiber array.    -   a pulse repetition rate may be greater than about 100 kHz.    -   control signals may be applied at a rate less than approximately        1/10th of a pulse repetition rate.    -   a pulse repetition rate may be less than about 100 kHz.    -   control signals may be generated from a leakage signal, for        example between pulses in individual fibers of the array.    -   an amplifier array may include a multicore fiber amplifier.    -   an amplifier array may include multiple multicore fiber        amplifiers.    -   a multicore fiber amplifier may include individual elements        constructed from step-index fiber, leakage channel fiber,        photonic crystal fiber or Bragg fibers.    -   individual fiber amplifiers may be polarization maintaining.    -   a phase plate may be inserted between the master oscillator and        a fiber amplifier array, so as to maximize coupling efficiency        of the master oscillator into each amplifier of the fiber        amplifier array.    -   a phase plate inserted downstream of an output of a amplifier        array may be included so as to maximize the Strehl ratio of the        output of the fiber    -   the system may include a means for side-pumping of the amplifier        arrays.    -   an amplifier may be constructed in a double pass configuration,        and may include a Faraday rotator inserted after the first pass.    -   an amplifier may be constructed in a single pass configuration.    -   phase control elements may be formed as an integrated array of        elements, for example as a MEMs or SLM.    -   phase-control elements may include portions of a a mirror array.    -   phase-control elements may include portions of a MEMs array.    -   phase-control elements may include portions of a liquid crystal        spatial beam modulator.    -   a phased pre-amplifier array may be included and matched in        optical dimensions to the amplifier array.    -   a pre-amplifier array may be tapered at its input end to        simplify coupling of the master oscillator to the pre-amplifier        array.    -   a fiber amplifier array may be spatially separated into        individual elements at its input and in optical contact at its        output end, so as to simplify coupling of the master oscillator        into the amplifier array.    -   a fiber amplifier array may be optically pumped with a single        pump source injected at the optical contact end of the amplifier        array.    -   a system may include a signal reference arm, and the reference        arm arranged to interfere with a fraction of an output beam of        the fiber array so as to facilitate detection of the optical        phase of each individual fiber amplifier output.    -   a fixed dither frequency may be used to control a plurality of        phase-control elements, for example phase modulators.    -   various dither frequencies may be used to control to a plurality        of phase-control elements.    -   the phase of a reference arm may be modulated at a fixed        frequency.    -   the system may include one or more detectors, for example a        detector array.    -   the system may include a cw laser coupled and transmitted        through each individual fiber amplifier of the array, and        configured for heterodyne phase detection and stabilization of        the optical phase of each of the fiber amplifiers of the array.    -   a control signal may be derived from a genetic algorithm        designed to maximize the Strehl ratio of an output of the fiber        array.    -   a fiber amplifier array may be used for EUV or X-ray generation.    -   a fiber amplifier array may be used as a light source in optical        lithography.    -   a fiber amplifier array may be used as a pump source for        parametric amplification.    -   a laser source may include a mode-locked oscillator.    -   the system may include a pulse stretching stage after the        oscillator, and a pulse compressor stage inserted downstream        from an array of fiber amplifiers.    -   a heterodyne phase detector and a plurality of phase-control        elements may be configured for maximizing the Strehl ratio of an        output of the fiber array.    -   a reference arm may be derived from an individual element of a        fiber array.    -   an array of modulators may be configured for modulating the        optical phase of the individual amplifiers at different        frequencies.    -   a heterodyne phase detector may be configured to maximize the        Strehl ratio of the output of the fiber array.    -   a spatial distribution of one or more pulses may be nearly        diffraction limited.    -   optical energy coupling between fiber array elements may be less        than about 1%.    -   relative fluctuations of array elements may be limited to less        than about 1 KHz.    -   a means for producing a plurality of control signals may include        a detector array and an adaptive algorithm for processing phase        information obtained from a detector.    -   an adaptive algorithm may include a genetic algorithm.    -   a means for seeding may include a mode-locked fiber oscillator.    -   a means for seeding may include a pulse stretcher for increasing        a pulse width of a pulse emitted from a mode locked laser or        other source.    -   a laser source may include a semiconductor laser diode, and        pulse widths may be produced in the range of picoseconds to        about one microsecond.    -   a fiber array may include a multi-core fiber.    -   a multi-core fiber may include a leakage channel fiber.    -   a leakage channel fiber may be polarization maintaining    -   control signals may be applied at a rate of approximately ten        times higher than a pulse repetition rate.    -   the system may include a signal reference arm and a means for        phase compensation, arranged to interfere with a fraction of a        beam passing backward through the fiber array, and allowing        compensation of the optical phase of each individual array        element for a beam passing forward through said fiber array with        the means for phase compensation.    -   a means for phase compensation may include a spatial beam        modulator, for example a commercially available spatial light        modulator (SLM).    -   energy coupling between fiber array elements may be less than        about 0.1%.    -   phase-control elements may be configured to modulate the phase        of said amplifiers.    -   a phase -control element may include a phase modulator.    -   the amplifier system may include a multicore leakage channel        fiber (LCF).    -   the array of amplifiers may be disposed about a common center        and approximately equidistant from said center.    -   a control bandwidth of a phase controller may be less than about        10 KHz.    -   the amplifier system may include a multicore fiber. The array of        amplifiers may be disposed in a single ring near the periphery        of said multicore fiber.    -   phase control elements may form a portion of an integrated phase        modulator.    -   a compact system configuration is obtainable as one result of        the low bandwidth phase fluctuations of the fiber outputs. For        example, phase control elements may be included with MEMs, SLMs,        micromirror arrays, or other integrated devices and/or        assemblies.    -   nearly diffraction limited outputs for pulse widths greater than        about 10 fs are obtainable as one result of phase compensation,        and as a result of limited mode coupling between fiber        amplifiers of the array.    -   an output pulse width may be in the range of about 100 fs to 1        ns, 100 fs to 10 ps, 1 ps to 1 ns, or about 100 ps to 50 ns.

The above description of the preferred embodiments has been given by wayof example. From the disclosure given, those skilled in the art will notonly understand the present invention and its attendant advantages, butwill also find apparent various changes and modifications to thestructures and methods disclosed. It is sought, therefore, to cover allsuch changes and modifications as fall within the spirit and scope ofthe invention, as defined by the appended claims, and equivalentsthereof.

What is claimed is:
 1. A high peak power fiber amplifier system,comprising: an array of fiber amplifiers in a multicore fiber, a spatialseparation of the cores of said amplifiers being sufficiently small toprovide strong thermal coupling therebetween which limits output phasefluctuations of the array to less than about 10 kHz, and sufficientlylarge to substantially limit optical mode coupling between amplifiers ofthe array, wherein said multicore fiber comprises a plurality of leakagechannel fibers having leakage channels disposed between the cores ofsaid amplifiers, wherein said leakage channel fibers are filled with airor a glass having a reduced refractive index; a plurality ofphase-control elements arranged in a spatial relation and opticallyconnected to fiber amplifiers of said array, said elements modifying anoptical phase of at least one fiber amplifier output in response to aphase control signal; and a phase controller generating said phasecontrol signal, wherein said phase control signal and said phase controlelements stabilize the optical output phase of the majority of saidindividual fiber amplifiers.
 2. The fiber amplifier system according toclaim 1, wherein said amplifier system comprises a multicore fiber, andsaid array is circular such that said amplifiers are disposed in asingle ring and approximately equidistant from a common center.
 3. Thehigh peak power amplifier system according to claim 1, wherein saidmulti-core fiber comprises a polarization maintaining fiber.
 4. Thefiber amplifier system according to claim 1, wherein said phase controlelements form a portion of an integrated phase modulator.
 5. The fiberamplifier system according to claim 1, further comprising: a lasersource for seeding said array of amplifiers and producing pulses havingpulse widths in the range of femtoseconds to about 1 microsecond; a beamdistributor disposed between said laser source and said array todistribute a pulse from said laser source into a plurality of beam pathsincident on corresponding amplifiers of the array, said beams having aspatial distribution substantially similar to the spatial distributionof said pulse source; and at least one pump source configured foroptically pumping said fiber amplifier array.
 6. The fiber amplifiersystem according to claim 1, wherein the multicore fiber is tapered atthe signal input end.
 7. The fiber amplifier system according to claim6, said system further comprising a phase plate inserted downstream ofthe output of said amplifier array, so as to optimize the Strehl ratioof the output of said fiber amplifier array.
 8. The fiber amplifiersystem according to claim 6, wherein said phase controller comprises adetector array and an adaptive algorithm for processing phaseinformation obtained from said detector.
 9. The fiber amplifier systemaccording to claim 5, wherein said laser source comprises a mode lockedlaser.
 10. The fiber amplifier system according to claim 5, wherein saidlaser source comprises a pulsed master oscillator seeding said fiberamplifier array.
 11. The fiber amplifier system according to claim 6,said system comprising a signal reference arm and a phase compensator,said reference arm being arranged to interfere with a fraction of a beampassing backward through said fiber array so as to provide compensationof the optical phase of each individual fiber amplifier for a beampassing forward through said fiber array with said phase compensator.12. The fiber amplifier system according to claim 11, wherein said phasecompensator comprises a spatial light modulator.
 13. The fiber amplifiersystem according to claim 6, said optical phase of the individualelements of said fiber amplifier array further being dithered at afrequency derived from a local oscillator so as to allow for heterodynephase detection of the optical phase of the majority of the elements ofsaid fiber array with a detector array, and said heterodyne phasedetector further configured for stabilizing the optical output phasebetween the majority of said individual elements of said fiber array.14. The fiber amplifier system according to claim 1, wherein the spatialseparation of the cores of the amplifier limit mode coupling to about0.1% or less.
 15. The fiber amplifier system according to claim 1,wherein energy coupling between amplifiers of said array of fiberamplifiers is less than 1%.
 16. The fiber amplifier system according toclaim 1, wherein said relative phase fluctuations are less than 1 kHz.17. The fiber amplifier system according to claim 1, wherein said phasecontroller comprises a detector array and an adaptive algorithm forprocessing phase information.
 18. The fiber amplifier system accordingto claim 1, wherein a phase-control element comprises a portion of asegmented mirror array.
 19. The fiber amplifier system according toclaim 1, wherein a phase-control element comprises a portion of a MEMsarray.
 20. The fiber amplifier system according to claim 1, said fiberamplifiers arranged such that optical energy coupling between any of thefiber amplifiers of said array of fiber amplifiers is negligible. 21.The fiber amplifier system according to claim 1, wherein said amplifieris operably arranged for coherent combination of sub-nanosecond laserpulses.
 22. The fiber amplifier system of claim 1, wherein nearlydiffraction limited outputs are obtainable for pulse widths greater thanabout 10 fs as a result of phase stabilization.
 23. A fiber amplifiersystem, comprising: an array of fiber amplifiers in a multicore fiber,wherein said multicore fiber is arranged such that thermal coupling ofindex of refraction fluctuations of the cores of said individual fiberamplifiers of said multicore fiber limits the bandwidth for coherentaddition of amplifier outputs, and wherein a spatial separation of thecores of said amplifiers is sufficiently large to substantially limitoptical mode coupling between amplifiers of the array, wherein saidmulticore fiber comprises a plurality of leakage channel fibers havingleakage channels disposed between the cores of said amplifiers, whereinsaid leakage channel fibers are filled with air or a glass having areduced refractive index; a plurality of phase-control elements arrangedin a spatial relation and optically connected to fiber amplifiers ofsaid array, said elements modifying an optical phase of at least onefiber amplifier output in response to a phase control signal; and aphase controller generating said phase control signal, wherein saidphase control signal and said phase control elements stabilize theoptical output phase of the majority of said individual fiberamplifiers.
 24. The fiber amplifier system according to claim 23, saidfiber amplifier system operably arranged such that a spatial separationof the cores of said amplifiers being sufficiently small to providestrong thermal coupling therebetween which limits output phasefluctuations of the array to less than about 10 kHz.
 25. The fiberamplifier system according to claim 23, wherein said spatial separationlimits energy coupling between amplifiers of said array of fiberamplifiers to less than 1%.
 26. The fiber amplifier system according toclaim 23, wherein said phase control elements are formed as anintegrated array of elements, including MEM and/or an SLM.